CRANIAL NERVES. 165 



color, and form gray rami communicantes for a part of their course to 

 the spinal nerves. 



The sympathetic system is best developed in the trunk, but it 

 extends forward into the head, where a series of sympathetic ganglia 

 (ciliary, sphenopalatine, etc.) is connected with the cranial nerves 

 as far forward as the fifth. The sympathetic trunk in this region is 

 usually closely connected with other nerves, but occasionally (Vidian 

 nerve from the sphenopalatine to the facial ganglion, Jacobson's 

 commissure from the seventh to the ninth, fig. 170) it is distinct. 



A few words may be added to this general account. In the elas- 

 mobranchs there is no sympathetic trunk, this first appearing in the 

 teleosts. The system is more highly developed in the aquatic than 

 in the terrestrial urodeles or in the anura. In the sauropsida the 

 trunk is usually double on either side in the neck region, .one branch 

 running through the vertebrarterial canal of the vertebras. In the 

 mammals the cervical part of the trunk is usually closely associated 

 with the pneumogastric nerve. In the development certain ganglion 

 cells migrate from the developing sympathetic system and pass to various 

 parts of the body, being usually closely associated with the glands of 

 so-called internal secretion hypophysis, carotid gland, suprarenals, 

 etc. They possess a peculiar affinity for chromic salts and are 

 known as chromaffine cells. Little is known of tbeir function. 



The Cranial Nerves. 



The nerves which arise from the brain and pass out through the 

 foramina in the skull are known as the cranial nerves. While in a 

 general way they resemble the spinal nerves, they have been specialized 

 and modified in many respects in correspondence with the specialization 

 of the head itself, some consisting of sensory fibres alone, some of only 

 motor fibres, while others are mixed, that is, contain both kinds of 

 fibres. There can also be recognized somatic and visceral nerves as in 

 the trunk, while the somatic sensory fibres may be arranged in dif- 

 ferent groups, differing in their connexions inside the brain and in the 

 sense organs to which they are distributed. Thus six different kinds of 

 fibres may occur in the cranial nerves, as follows: 



1. The somatic motor, which go to the muscles derived from the 

 myotomes of the head. 



2. The visceral motor, distributed to the muscles of the gill region, 



